1. Technical Field
The present invention relates to a dye-sensitized solar cell, specifically, to a dye-sensitized solar cell which is designed to reduce the production cost, improve productivity and increase energy efficiency, by using a carbon electrode as a counter electrode, and a manufacturing method thereof.
2. Description of the Related Art
As it has been well known in this field of art, a solar cell is a device that converts solar energy into electricity. It uses two types of semiconductors, P type and N type semiconductor for power generation. When placed under the sunlight, electrons and holes are generated in the solar cell, and the generated electrical charges transport to P-type side and N-type side, causing a potential difference between the P-type and N-type side.
In recent years, researches in dye-sensitized solar cells based on the principle of a photosynthetic reaction have been made. Such dye-sensitized solar cell is a novel type of photoelectrochemical solar cells with enhanced energy conversion efficiency by chemically adsorbing photosensitive dye molecules to the surface of a semiconductor material having a large energy bandgap, unlike a silicon solar cell, wherein the photosensitive dye molecules can generate electron-hole pairs when receiving light within the visible light area. Such dye-sensitized solar cells, as compared to conventional solar cells using compound semiconductors, require low manufacturing cost and a simple process, while having another advantage such as being readily applicable to the window of the exterior wall in a building, to a green house made of glass or the like, owing to the use of eco-friendly and transparent electrode. However, such dye-sensitized solar cells still have drawbacks with regard to physical durability and limits in photoelectric transformation efficiency.
As representative dye-sensitized solar cells, there are “photo-electrochemical cells” shown in U.S. Pat. No. 4,927,721 issued on the date of May 22, 1990 to Gratzel et al. (Switzland), and “photovoltaic cells” shown in U.S. Pat. No. 5,350,644 issued on the date of Sep. 27, 1994 to the same applicant.
Such conventional dye-sensitized solar cells comprise a working electrode, a counter electrode and an electrolyte intervened therebetween.
The working electrode comprises a conductive transparent electrode formed on one side of a transparent substrate; a layer of n-type metal oxide semiconductor as a nano-porous membrane formed on the conductive transparent electrode; and a dye layer coated on the titanium oxide layer.
The counter electrode is separated from the working electrode by the electrolytic layer, and made of precious metals such as platinum, palladium, gold, silver and the like by being formed on the other side of the transparent substrate.
By such construction, the dye-sensitized solar cells operate as follows: upon absorption of sunlight, the dye is excited and oxidized, from which electrons are provided to the conduction band of an n-type metal oxide semiconductor layer having a large bandgap. The empty space in the lower energy level made by the loss of electrons therefrom, is again filled with other electrons received from ions in the electrolyte. The ions which gave electrons to the dye move to the counter electrode for receiving electrons. At that time, the counter electrode works as a catalyst in oxidation-reduction reactions of ions in the electrolyte, thereby playing a role of providing electrons to the ions in the electrolyte through oxidation-reduction reactions occurred on the surface (the oxidized dye returns to the ground level as being reduced by obtaining electrons from an electron donor, I− present in the electrolyte, and the oxidation-reduction medium which was converted to I3− by donating electrons to such reaction is again converted to an electron donor, I− with the help of a counter electrode). For satisfying all the characteristics, conventional dye-sensitized solar cells have mostly used a platinum thin film which has excellent catalyst activity, as a counter electrode. Other than platinum, precious metal species which have similar characteristics to platinum have been used for the metal electrode, for example palladium, gold and silver.
However, the conventional dye-sensitized solar cells require high production cost owing to the use of a precious metal electrode such as platinum which has relatively high electroconductivity and excellent catalyst properties, as a counter electrode. Further, they have limitations on increasing the surface area for catalytic reactions being occurred.
When it is intended to use an insulating substrate, for example ceramic as a transparent substrate, a counter electrode should have more thickness in order to satisfy the demanded electroconductivity, therefore it requires the use of expensive equipment such as a large sputtering system or involves in a screen printing method for the manufacture, thus increasing the manufacturing cost, i.e. posing a problem of remarkable decrease in economic efficiency. Particularly, parameters such as the surface area and volume of a counter electrode have a significant relation with the rate of a catalyst reaction.